HP Workstation xw6000 Hardware Reference Manual page 56

Hardware Upgrades
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e. Place the thermal grease on the bottom of the heatsink before
setting the heatsink on top of the processor.
Your workstation may come preinstalled with screws around the
second processor area where the new heatsink will attach. Remove
these screws before installing the new heatsink.
f. Line up the captive screws with the holes on the system
board.
g. Tighten the screws 3 until they no longer turn and the spring
is fully compressed.
Installing a second processor/heatsink assembly
Hardware Reference Guide